Light straight-run or virgin naphtha is a hydrocarbon refinery process stream comprising pentane and hexane paraffins and is useful as a blending stock for motor fuels such as gasoline. However, the Research Octane Number of this hydrocarbon fraction is relatively low, generally in the range of 68-72. In the past, the octane of this fraction was conveniently raised to about 85-90 RON by the addition of alkyl lead compounds. More recently due to lead phase-down and aromatics reduction, refiners have implemented other means such as isomerization to improve the octane of this fraction. Isomerization processes typically produce a product having an octane of about 78-84 RON depending upon the temperature of the reaction. When the isomerization processes are integrated with separation processes such as adsorption or distillation, which separate the effluent from the isomerization reactor into higher and lower octane segments, the final product from the integrated process can have an octane of about 90 RON or greater.
Molecular sieve adsorbents have been utilized in a variety of processes in the hydrocarbon processing industry. One area of particular importance is in the above-mentioned field of octane upgrading, where hydrocarbon feedstocks containing pentane and hexane paraffin fractions are separated into high and low octane segments. In general, the normal paraffins and mono-methyl branched hexane paraffins comprise the low octane segment and the more highly branched paraffins (e.g., di-methyl paraffins), naphthenes, and aromatics comprise the high octane segment. However, isopentane has a high octane even though it has a structure similar to that of the mono-methyl hexane branched paraffins. Accordingly, because currently existing adsorption processes cannot conveniently separate isopentane and other high octane more highly branched paraffins from the lower octane normal and mono-methyl branched hexane paraffins, valuable high octane components are often lost when upgrading the octane of a hydrocarbon feed by adsorption.
In addition to the adsorption or separation function, most of the processes used for octane upgrading also incorporate an isomerization process that is used to further isomerize the low octane normal and mono-methyl branched hexane paraffins to higher octane isopentane and more highly branched hexane paraffins. Some or all of the effluent from the isomerization process is typically recycled back to the adsorption process for separation. Alternately, it can be combined with effluent from the adsorption process to form a combined total product.
The adsorption processes known in the art are generally of two types. One type performs a separation of normal from non-normal paraffins using an adsorbent commonly known in the industry as 5A or calcium zeolite A. This process is useful because it can process a hydrocarbon feed containing pentane and hexane paraffin fractions. While there are several variations of this type of process, it has been generally described in U.S. Pat. No. 4,210,771, issued to Holcombe, at col. 3, lines 18-34 as follows:
"In a broad aspect, the invention provides an integrated TIP process for improving the octane rating of a mixed hydrocarbon feedstock containing saturated paraffins having from 5 to 6 carbon atoms, which comprises passing said feedstock through an isomerization reactor containing a catalyst composition and a hydrogenation component in the presence of hydrogen to convert at least a portion of the normal hydrocarbons in the feedstock to non-normals; the hydrocarbons in the effluent from the reactor are passed to the adsorption section of the system where the normals are adsorbed in a molecular sieve zeolite adsorber bed and the non-normals are eventually passed out of the system as an isomerate product. The adsorber beds, after the adsorption cycle, are desorbed by a hydrogen purge gas producing a desorption vapor effluent containing desorbed normals and hydrogen purge gas." PA0 ". . . a tectosilicate having precise channel dimensions intermediate the channel dimensions present in either the calcium 5A sieve or the ZSM-5 sieve. The molecular sieve of this invention is capable of adsorbing not only normal hexane, but methyl pentanes as well. A preferred molecular sieve of this invention is a tectosilicate having channel dimensions intermediate 5.5.times.5.5 and 4.5.times.4.5, but excluding 4.5.times.4.5 (i.e. calcium 5A) Angstroms." (See col. 5, lines 42-50).
Another process employing similar adsorption steps is disclosed in U.S. Pat. No. 4,709,116, issued to Zarchy et al. While this type of process provides substantial benefits, the improvement in octane rating of the product is limited due to the presence in the non-adsorbed fraction of low octane mono-methyl branched hexane paraffins which are not readily adsorbed by the 5A zeolite.
The other type of adsorption process incorporates an adsorbent that has a slightly larger pore size which allows both normal paraffins and mono-methyl paraffins to be adsorbed but excludes the larger di-methyl branched paraffins. U.S. Pat. No. 4,717,784, issued to Stem et al., at col. 3, lines 59, et seq., describes an adsorption and isomerization process that upgrades the octane of a C.sub.6 (hexane) paraffinic feed by isomerizing the feed and subsequently separating the unreacted normal paraffins and mono-methyl branched paraffins from the di-methyl branched paraffins. This process, however, fails to make any separation of the relatively high octane mono-methyl paraffins having no more than 5 carbon atoms, e.g., isopentane, from the other adsorbed hexane and heavier mono-methyl paraffins, e.g., 2-methyl pentane, and normal paraffin species. All adsorbed mono-methyl paraffins are desorbed along with the normal paraffins and recycled to the isomerizer. The molecular sieve disclosed in U.S. Pat. No. 4,717,784 is described as:
A most preferred molecular sieve, according to this patent, is ferrierite. U.S. Pat. Nos. 4,804,802, issued to Evans et al., and 4,855,529, issued to Stem et al., provide similar disclosures to U.S. Pat. No. 4,717,784 regarding the adsorbent suitable for adsorbing mono-methyl branched hexane paraffins and also disclose the use of additional adsorbents, e.g., 5A, for adsorbing normal paraffins. The patents, however, fail to provide a solution to the problem of how to treat feeds containing isopentane which is also a mono-methyl paraffin and is desired as a high octane product unlike the mono-methyl branched hexane paraffins which have a low octane.
It can be appreciated that in light of the two types of processes described above, processes are sought which can upgrade the octane of a hydrocarbon feedstock containing pentane and hexane paraffinic fractions by separating the low octane normal and mono-methyl branched hexane paraffins from the higher octane, isopentane and more highly branched hexane paraffins.
In addition to the calcium 5A and ferrierite molecular sieves proposed for use in the octane upgrading processes, other molecular sieves have been proposed to perform separations in other hydrocarbon ranges. For example, in the area of aromatics production zeolites of the type ZSM-5, ZSM-11, ZSM-23, and ZSM-35 have been proposed to separate normal and methyl branched paraffins and olefins from aromatics compounds. See U.S. Pat. No. 4,423,280, issued to Dessau. U.S. Pat. No. 4,448,671, issued to Dessau, discloses the use of the same adsorbents to separate waxy linear and methyl-branched paraffins from other paraffinic compounds such as, for example, crude oils, heavy oils, distillate oils and lube base oil stocks.
U.S. Pat. Nos. 4,367,364 and 4,455,444, issued to Kulprathipanja et al., disclose processes whereby normal paraffins are separated from cyclic and branched chain hydrocarbons by contacting the feed with an adsorbent comprising silicalite. These processes operate at conditions that allow normal paraffins, but not branched chain hydrocarbons, to be adsorbed. U.S. Pat. No. 4,455,444, discloses an example which illustrates that the normal paraffins can be recovered in order of increasing molecular weight, from n--C.sub.10 to n--C.sub.15, by purging with a displacement fluid. The use of the displacement fluid, and subsequent elution of the normal paraffins, is analogous to the use of a carrier fluid in chromatography.
Silicalite has also been proposed for use in column chromatography to isolate mono-methyl alkanes from complex hydrocarbon mixtures. T. C. Hoering and D. H. Freeman, Journal of Chromatography, 316 (1984) 333-341, disclose adsorbing mono-methyl alkanes on silicalite and then recovering mono-methyl alkanes by desorbing with a normal alkane such as normal octane. The results of the study indicated that silicalite could be used to chromatographically separate methyl alkanes according to their relative adsorptivities in the same way that smaller pore calcium zeolite A could be used to separate normal alkanes according to their relative adsorptivities.